The invention relates to the determination of the angle of peak amplitude response of a radiation-sensitive resonant device, and more particularly to facilities and techniques for determining the angle of peak reflection, from a prescribed internal atomic plane of a crystal, of an X-ray beam incident on the atomic plane through a reference surface of the crystal as the reference surface is scanned by such beam.
Highly accurate electronic frequency sources employ cut piezoelectric crystals, such as quartz crystals, to determine a unique, ultrastable reference frequency. Such cut crystals have a planar reference surface which, for each desired reference angle, is cut at a predetermined angle with respect to at least one atomic plane within such crystal. (The angle between the reference surface and the atomic plane defines what is commonly known in the art of AT-cut quartz crystals as the "Z--Z' angle"). Since differences in such angle will produce corresponding differences in the temperature-frequency characteristics of the crystal, it is important when mass-producing such crystals to determine rapidly and accurately the value of such angle in order to sort the successive crystals into batches corresponding to the different temperature characteristics.
Present automated arrangements for accomplishing this function employ an X-ray diffraction technique, whereby the test crystals are positioned on a crystal mount associated with a rotating spindle of a goniometer. The crystal is mounted for rotation about an axis parallel to the reference plane, and such surface is scanned by rotating the spindle through a prescribed angle with respect to a highly collimated, X-ray beam which is incident on the atomic plane of the crystal through the reference surface. The angle of scan includes the range encompassing the resonant-type response characteristic of the crystal.
The resulting radiation is reflected from the atomic plane via the reference surface, and the accumulated pulses picked up by the detector correspond to the amplitude response of the crystal at that point of the scan.
During the rotation of the spindle, a shaft encoder coupled thereto indicates the attained angle of scan as the detector accumulates the reflected pulses. The output of the detector is coupled to a peak sensing circuit. As the crystal is scanned over the peak region of the crystal response, the peak sensing circuit sends a triggering signal to the shaft encoder which outpulses a digital quantity indicative of the peak angle at which the detector response is optimum, and such angle is recorded automatically or manually. The outpulsed digital signal from the encoder also operates a suitable sorting device which discharges the tested crystal into an appropriate bin corresponding to the attained angle of the peak response for the purposes indicated above. Alternatively, when the measured peak angle falls outside a predetermined range, the crystal may be rejected.
Such existing types of automated crystal inspection apparatus have several disadvantages, which are related principally to the fact that the X-ray diffraction technique employed is statistical in nature. That is, as the reference surface of the crystal is angularly scanned over the resonant characteristic, the multiplier-type detection tube counts and integrates the number of essentially random pulses that are reflected toward such detector from the atomic plane of the crystal as an indication of reflected intensity. In general, the multiplier tube output does not reach a steady state before the crystal mount advances beyond the associated angle; and as a result, the range of statistical error in the integrated pulse count at each attained angle of scan can overlap the integrated pulse count at the preceding and succeeding angles of scan, thereby leading to a multi-valued rather than a monotonic measured characteristic.
The statistical errors indicated above have relatively little effect when measurements are taken on a crystal having a sharp reflected intensity characteristic, since even in the peak regions of such characteristic the mean differences in the accumulated number of reflected pulses detected between two points of attained angle during the scan are large enough to exceed the normal range of statistical error at any given point in the scan. Thus, when the peak portion of the curve is sharp, i.e., when the test crystal has already undergone a succession of expensive lapping and polishing operations after its initial cut from a mother crystal, the desired angle of peak response of the device can be determined with acceptable accuracy, since the successive measured points around the sharp peak differ sufficiently in amplitude from each other to fall outside the statistical error.
However, when such peak is relatively broad, as is the case when testing crystals in the early stages of processing where their rejection would entail relatively small expense, the existing peak-seeking techniques are inaccurate, since the output of the scintillation counter in the region about the peak will constantly be subjected to an error greater than the mean difference in amplitude of successive points around the peak. As a result, the angle read-out by the shaft encoder when triggered by the peak detector will be unpredictable as an indication of the desired peak angle.